The long-term goal of this research is to characterize and mitigate a new phenomenon involving commensal enteric bacteria that catabolize and inactivate anthelmintics drugs used to treat gastrointestinal nematodes. The specific objective of this application is to genotype the commensal bacteria and to determine whether the phenomenon diminishes anthelmintic effectiveness in a model gastrointestinal nematode. Until we address the gap in knowledge concerning this emerging phenomenon, gastrointestinal nematode treatment failures will continue to rise and benzimidazoles may become obsolete. Preliminary studies characterized commensal gut bacteria (family Enterobacteriaceae) isolates that putatively catabolize benzimidazoles, thus decreasing benzimidazole efficacy when the bacteria co-existed with a free- living nematode. Our central hypothesis is that these commensal gastrointestinal Enterobacteriaceae strains contribute to drug failures against gastrointestinal nematodes. The rationale underlying the proposed research is that anti-nematodal drug resistance is on the rise, and our evidence suggests that these commensal gut bacteria contribute to this problem. It is therefore prudent to determine genotypes of the bacteria involved in this phenomenon, and to extrapolate our preliminary data to nematodes that infect humans. Our preliminary studies indicate that the catabolic phenotype is encoded on selfish and transferrable plasmids, thus its global dissemination is likely. Guided by strong preliminary data, we plan to objectively test our central hypothesis and attain the objective of this application by pursuing the following two specific aims: 1) To identify the Enterobacteriaceae and their genetic elements conferring the benzimidazole catabolism; and, 2) To assess the ability of benzimidazole-catabolizing Enterobacteriaceae to protect adult gastrointestinal nematodes from the effects of albendazole. Under the first aim, we will identify the benzimidazole-catabolizing genes and the Enterobacteriaceae bearing these genes. Under the second aim, we will assess the efficacy (or lack thereof) of albendazole on Ascaris suum co-incubated with Enterobacteriaceae that catabolize albendazole. The project is innovative because it will be the first to characterize this emerging problem involving anti-nematodal drug failures and bacteria that naturally co-exist with gastrointestinal nematodes. The proposed research is significant because it will characterize a novel cooperative interaction between gastrointestinal nematodes and Enterobacteriaceae, an interaction that will complicate nematocidal therapies. Ultimately, such knowledge will form the basis for mitigation strategies that pre-empt the cooperativity. For instance, we may be able to find non-substrate benzimidazoles or potentiators that block the catabolic enzymes.